During freefall skydiving, the determination of one's altitude above the local terrain (above ground level or AGL) is of primary importance in order to deploy a main parachute at a sufficiently high altitude to allow for a safe cutaway, or jettisoning of the main parachute, and full reserve deployment in case of a main parachute malfunction, During what is known as relative work or formation flying, where anywhere from two to two hundred people may be flying in close proximity, it is of equal importance to know when to cease flying in information in order to have time to gain safe horizontal separation from one another before deploying a parachute. Precise altitude information is usually provided by a mechanical dial-face altimeter, worn on the chest or back of hand. However, it is impractical and dangerous to continuously monitor one's altimeter in flight, due to the need to constantly maintain awareness of the locations and sometimes hazardous flight trajectories of those around you in order to avoid potentially fatal freefall collisions. For experienced competition teams of, for example, eight people, the collision hazard is small, but the concentration is on maintaining eye contact in order to fly " relative" and perform different maneuvers, taking and releasing handholds ("grips") on your teammates' arms or legs, a process known as turning points. To turn the maximum number of points is the goal, and the better the level of eye contact, the more points can be turned. This is at odds with the need to glance at a chest-mounted or wrist-mounted altimeter or even the ground; intense concentration on the turning of points can cause loss of time and altitude awareness. The only solution until now is the use of an audible alert, which may be used but which has disadvantages as outlined below.